1. Field of the Invention
The present invention relates to a liquid crystal display device and in particular to a vertically oriented liquid crystal display device excellent in characteristics of viewing angle.
2. Description of the Related Art
A liquid crystal display device comprises a liquid crystal cell and a polarizing plate. The foregoing polarizing plate is provided with a protective film and a polarizing film, and can be obtained by staining with iodine a polarizing film made of, for example, polyvinyl alcohol film, stretching the resulting film and laminating both sides with protective films. In a transmission-type liquid crystal display device, the polarizing plates may be fixed on both sides of the liquid crystal cell and one or more optically compensated sheets (optically anisotropic layers) may also be disposed thereto. In a reflection-type liquid crystal display device, components are disposed in the order of a reflector plate, a liquid crystal cell, one or more optically compensated sheets and a polarizing plate. The liquid crystal cell consists of liquid crystal molecules, two substrates for sealing them and an electrode layer for energizing the liquid crystal molecules. Display modes such as TN (Twisted nematic), IPS (In-Plane Switching), OCB (Optically Compensatory Bend), VA (Vertically Aligned) and ECB (Electrically Controlled Birefringence) for which an ON/OFF display is carried out due to a difference in the orienattion status of liuid crystal molecules and can be applied either to a transmission-type or a reflection-type system are proposed.
Among these liquid crystal displays (LCD), nematic liquid crystal molecules having positive dielectric anisotropy are used in applications where a high display quality is needed. More particularly, a 90-degree twisted nematic-type liquid crystal display device driven by a thin layer transistor (hereinafter referred to as TN mode) is mainly used. Although, the TN mode has excellent display properties when viewed from the front, it also has problematic characteristics of viewing angle such as a reduction in contrast when viewed in the oblique direction and deterioration in display properties due to occurrence of tone reversal in which brightness is reversed by gradation display, therefore, improvement has strongly demanded.
A vertical oriented nematic-type liquid crystal display device (hereinafter referred to as VA mode) has been proposed in recent years in which nematic liquid crystal molecules having negative dielectric anisotropy are used to allow the long axes of liquid crystal molecules to orient approximately vertical to a substrate, with no voltage applied, and these crystal molecules are driven by a thin-layer transistor as an LCD system for improving these characteristics of viewing angle (see, for example, Japanese Patent Laid-open Publication (KOKAI, hence force referred to as “JP-A”) No. 2-176625). The VA mode is not only excellent in display properties when viewed from the front, as with the TN mode, but also presents a wider viewing angle through application of a retardation film for compensating the viewing angle. It is also known that the VA mode is capable of providing a wider field-angle characteristic by using two negative monoaxial retardation films having the optical axis in the direction orthogonal to a film plane above and below the liquid crystal cell, and a monoaxial retardation film having a positive refractive-index anisotropy with the in-plane retardation value of 50 nm is being used in this LCD to attain a wider field-angle characteristic as well (see SID 97 DIGEST from page 845 to 848).
However, use of the three retardation films would not only increase production costs but also decrease the yield for laminating a mass of films, thereby resulting in an increase in thickness of a plurality of films and posing problems in making a display device thinner. In addition, since a pressure-sensitive adhesive layer is used for laminating stretched films, the pressure-sensitive adhesive layer may contract due to a change in temperature or moisture, thus resulting in defects such as peeling between the films and warpage.
Methods for improving these defects including a method for reducing the number of retardation films and a method for using cholesteric liquid crystal layers have been disclosed (see JP-A-11-95208). However, these methods also have required laminating a plurality of films, and have not been sufficient in terms of making the device thinner or reducing the production costs.
In addition, proposed is a method in which discotic liquid crystal compounds are used for the purpose of improving the above problems (see JP-A-11-352328). This method is, however, difficult in provision of a longitudinal optical film with an optically anisotropic layer and lower in productivity. A method for solving this problem has also been disclosed (see JP-A-2000-304931).
These retardation films have problems that their basic properties may vary greatly depending on changes in the storage environment. More particularly, it has been found that any change in temperature and humidity may increase or decrease retardation in the thickness direction, thereby reducing the effect of widening the viewing angle.
Proposed is a VA optical film with less temperature/humidity dependence as a method for solving the change in the storage environment (see JP-A-2001-114914). The film is assuredly smaller in temperature dependence and less influenced by the variation in the Rth but not sufficiently effective in widening the viewing angle (fundamental performance). Therefore, what has been requested is a VA retardation film which is sufficiently effective in widening the viewing angle as a fundamental performance and at the same time less affected in fundamental performance due to environmental change.
The present invention is to provide a liquid crystal display device wherein liquid crystal cells are optically compensated accurately and smaller in the number of the films to be laminated, thereby successfully making the device thinner and, in particular, a liquid crystal display device (VA mode) which is less affected in the fundamental performance due to any changes in the storage environment.